A typical level shifter is designed for level-shifting a low-voltage logic level to another low-voltage logic level, such as from a transistor-transistor logic (TTL) logic level to a complementary metal-oxide-semiconductor (CMOS) logic level, and/or vice versa; and frequently, both the digital input signal and the digital output signal are referenced to an identical ground potential. In contrast, there are some applications wherein a level shifter is required to level-shift a digital input signal referenced to an input ground potential to a digital output signal referenced to an output ground potential which may be either substantially higher or substantially lower than the input ground potential, and the following are several prior-art methods for designing level shifters for these applications.
One of the methods is based on pull-up resistors: the advantages include reasonably low cost; the disadvantages include dependence of ground-potential differential on transistor voltage ratings, more substantial propagation delays as ground-potential differential increases, a large mismatch of propagation delays among a plurality of level-shifter channels with varying ground-potential differentials, significant power consumption, intolerance of substantial slew rate of output ground potential relative to input ground potential, and a low signal frequency bandwidth.
Another one of the methods is based on optocouplers: the advantages include reasonably low cost, being scalable to high-voltage level-shifting, tolerance of high slew rate of output ground potential relative to input ground potential, and a reasonably wide signal frequency bandwidth; the disadvantages include relatively significant propagation delays, and subsequently a substantial mismatch of propagation delays among a plurality of level-shifter channels, and significant power consumption.
Still another one of the methods is based on digital isolators: the advantages include being scalable to high-voltage level-shifting, tolerance of high slew rate of output ground potential relative to input ground potential, reasonably short propagation delays, a reasonably good match of propagation delays among a plurality of level-shifter channels, and a reasonably wide signal frequency bandwidth; the disadvantages include high complexity and high cost, and significant power consumption.
Still another one of the methods is based on pulse transformers: the advantages include being scalable to high-voltage level-shifting, tolerance of substantial slew rate of output ground potential relative to input ground potential, and reasonably short propagation delays; the disadvantages include high cost, substantial size and weight, substantial power consumption, and a narrow signal frequency bandwidth.
Still another one of the methods is based on bootstrap high-side gate drivers: the advantages include being scalable to high-voltage level-shifting of up to hundreds of volts, tolerance of high slew rate of output ground potential relative to input ground potential, and a reasonable signal frequency bandwidth; the disadvantages include high complexity and high cost, significant propagation delays, a requirement of output ground potential being higher than input ground potential, significant power consumption, limited output voltage range, and a significant mismatch of propagation delays among a plurality of level-shifter channels.